Babies can begin eating toast by age 6 months once they start solid foods.
However you should cut the toast up in strips so that the baby can more easily chew the toast but giving the baby a whole piece of toast is okay as long as you're supervising the baby which you should be doing anyway.
Babies that are at least 6 months of age or older can have butter on toast or toast by itself.
Just make sure to cut the toast into small strips so that the baby can chew and eat the butter on toast or toast more easily and to avoid choking.
Once a baby reaches 6 months of age and above and is on solid foods the baby can eat anything except whole nuts, honey and raw shellfish.
Always make sure your baby is seated in a highchair and strapped in to avoid any choking hazard and always supervise the baby while they are eating.
I have fed my babies butter on toast my make sure to spread the butter out so that it too is not a choking hazard.
The butter should not be in clumps though but spread on the toast the butter on toast is perfectly fine for babies 6 months of age or older.
If the baby is under 6 months of age then they should not have any toaster whether butter on toast or regular toast.